Comparative studies over the last decade have shown that embryonic development among animals depends to an amazing extent on the same genes and interacting sets of genes. This realization has resulted in an experimental focus on he hypothesis that changes in utilization of these modular components have been the driving force in morphological volution. Because insects differ significantly in the manner in which their embryos segment, and because we understand more about the regulation of segmentation in Drosophila melanogaster than any other animal, insects have been a popular group for study. The approach applied to date, termed "comparative molecular embryology", has involved cloning from other insects, orthologs of Drosophila genes important to segmentation and examining their patterns of expression. Such studies suggest that, at least among holometabolous insects, common zygotically-expressed genes are important despite marked differences in the way various species segment. However, two factors intrinsically limit such an approach. Firstly, it is based on the premise that a common expression pattern implies a common function. In Tribolium castaneum, an ortholog of the Drosophila segmentation gene fushi tarazu is expressed during embryogenesis in a pattern resembling that of the fly gene, but appears dispensable for segmentation. Thus, in the single case where this premise has been tested, it is not fulfilled. Secondly, this approach only allows an examination of the possible significance in other insects of mechanism described for Drosophila, but cannot discover regulatory events important for other insects but not in flies. The proposed research will address these shortcomings through studies of Tribolium which is unique among nonDrosophilid insects in allowing facile genetic analysis as well as molecular and developmental methodologies. Two objectives are paramount. Firstly, although orthologs of pair-rule genes resemble Drosophila in showing a striped expression pattern, their registers and/or extents differ in ways likely to be functionally significant. (If Drosophila orthologs are expressed in Tribolium patterns, embryological defects would result in each case.) We will screen for mutations in the ribolium pair-rule genes in order to assess their overall importance to segmentation, as well as possible differences between flies and beetles in their regulative interactions. Secondly, we will study mutations isolated in direct screens for genes important to embryonic patterning in Tribolium. In addition, we will assess the presence and possible role of a ribolium ortholog of bicoid, the maternally-expressed anterior determinant in Drosophila, as well as examine the possible conservation of regulatory interactions affecting the fushi tarazu ortholog. Because of the background available for drosophila and the genetic capabilities of the Tribolium system, this research provides a unique opportunity to describe the evolution of a complex regulatory hierarchy.